Technical Field
The present invention relates to a semiconductor device and a method of manufacturing the semiconductor device.
Background Art
Silicon (Si) has conventionally been used as the material for power semiconductor devices for controlling high voltages and high currents. There are various types of power semiconductor devices, such as bipolar transistors, insulated-gate bipolar transistors (IGBTs), and metal-oxide-semiconductor field-effect transistors (MOSFETs), and these devices are used for different purposes according to the use case.
For example, bipolar transistors and IGBTs have a higher current density than MOSFETs and make it possible to work with higher currents but cannot be switched at high speeds. More specifically, bipolar transistors are limited to being used at switching frequencies on the order of several kHz, and IGBTs are limited to being used at switching frequencies on the order of several dozen kHz. Power MOSFETs, on the other hand, have a lower current density than bipolar transistors and IGBTs and make it difficult to work with high currents but can be operated at high switching speeds on the order of several MHz.
However, there is strong commercial demand for power semiconductor devices that can both handle large currents and offer good high speed performance. Much effort has been focused on improving IGBTs and power MOSFETs, and currently, these devices have been developed to near the performance limits of the materials being used. There is various ongoing research on semiconductor materials that can replace silicon in power semiconductor devices, and silicon carbide (SiC) has attracted attention as a semiconductor material that could potentially make it possible to manufacture next-generation power semiconductor devices with low on-voltages and excellent high speed and high temperature performance (see Non-Patent Document 1).
Silicon carbide is a semiconductor material with exceptional chemical stability that also has a wide bandgap of 3 eV and can be used as a semiconductor in an extremely stable manner even at high temperatures. Moreover, silicon carbide has a maximum field strength of at least an order of magnitude greater than that of silicon and therefore shows potential as a semiconductor material that could make it possible to make the on-resistance of a device sufficiently small. These advantageous properties of silicon carbide are also exhibited by other wide-bandgap semiconductors that have a wider bandgap than silicon, such as gallium nitride (GaN). Therefore, using wide-bandgap semiconductors will make it possible to increase the breakdown voltage of semiconductor devices (see Non-Patent Document 2, for example).
Due to the reduction in incurred losses that comes with using silicon carbide in high breakdown voltage semiconductor devices, using such devices in an inverter makes it possible to operate at carrier frequencies an order of magnitude greater than with conventional semiconductor devices made using silicon. Operating a semiconductor device at high frequencies increases the temperature of the chip due to the resulting heat generation and affects the reliability of the semiconductor device. Particularly in a semiconductor device in which bonding wires are bonded to a front surface electrode on the front surface side of a substrate as a wiring material for extracting voltage from the front surface electrode to outside of the device, when the device is used at high temperatures, the adhesion between the front surface electrode and the bonding wires decreases, thereby affecting the reliability of the device.
There are also technologies in which sheet-shaped conductors are used as a wiring material other than bonding wires for extracting voltage from the front surface electrode to outside of the device (see Patent Document 1, for example).
Furthermore, there are conventional silicon carbide semiconductor devices in which pin electrodes are bonded to the front surface electrode with solder in order to reduce any decrease in adhesion between the front surface electrode and the bonding wires. FIG. 5 is a cross-sectional view illustrating a configuration of a conventional silicon carbide semiconductor device. An n-type silicon carbide epitaxial layer 2 is deposited onto the surface of an n+ silicon carbide substrate 1, and a plurality of p+ regions 10 are formed in the surface of the n-type silicon carbide epitaxial layer 2. A p-type silicon carbide epitaxial layer 11 is formed on the surfaces of the p+ regions 10. n-type well regions 12 are formed in the p-type silicon carbide epitaxial layer 11 on the n-type silicon carbide epitaxial layer 2 in areas in which the p+ regions 10 are not formed. n+ source regions 4 and p++ contact regions 5 are formed in the surface of the p-type silicon carbide epitaxial layer 11.
Gate electrodes 7 are formed on the surface of the portions of the p-type silicon carbide epitaxial layer 11 that are sandwiched between the n+ source regions 4 and the n-type well regions 12, with a gate insulating film 6 interposed therebetween. An interlayer insulating film 14 is selectively formed on the gate electrodes 7. A source electrode 8 is formed over the surfaces of the n+ source regions 4 and the p++ contact regions 5. A protective film 15 is selectively formed on the source electrode 8, and a plating film 16 is formed in the areas in which the protective film 15 is not formed.
A second protective film 17 is formed covering the portions where the plating film 16 and the protective film 15 are adjacent to one another. A pin electrode 18 is formed connected to the plating film 16 via solder 19. A drain electrode 9 is formed on the rear surface side of the n+ silicon carbide substrate 1.
In the MOSFET structure illustrated in FIG. 5, when a positive voltage relative to the source electrode 8 is applied to the drain electrode 9 and a voltage less than the gate threshold value is applied to the gate electrodes 7, the p-n junctions between the p-type silicon carbide epitaxial layer 11 and the n-type well regions 12 become reverse biased, and current does not flow because the breakdown voltage of the active regions is maintained. Meanwhile, when a voltage greater than or equal to the gate threshold value is applied to the gate electrodes 7, current flows due to the formation of inversion layers in the surface of the p-type silicon carbide epitaxial layer 11 directly beneath the gate electrodes 7, therefore making it possible to switch the MOSFET ON and OFF according to the voltage applied to the gate electrodes 7.